The electric power steering device for vehicle detects the steering torque generated on the steering shaft by the operation of a steering wheel and the speed of vehicle, computes the motor current reference value based on the signal detected thereby, drives the motor with current corresponding to the motor current reference value computed thereby to assist the steering power of the steering wheel. Such an electric power steering device is controlled by an electronic control circuit, and the control is summarized as follows.
Specifically, when the steering wheel is steered and a steering torque is generated, the electronic control circuit can give the optimum steering assist force corresponding to the running condition by computing the motor current reference value as constituting the target value of the motor output in response to the steering torque and the vehicle speed so that a large steering assist force may be supplied when the detected vehicle speed is zero or low and a small steering assist force may be supplied when the detected vehicle speed is high, and by proceeding to a current feedback control so that the difference between the motor current reference value resulting from the computation and the detected motor current value may be reduced to zero.
The above-mentioned control circuit of the electric power steering device computes the motor current reference value by taking the steering torque and the vehicle speed as parameters and does not take into account the rotation speed of the motor. The rotation of the motor leads to the generation of back electromotive force. However, if the impact of back electromotive force resulting from the rotation of the motor is not taken into account in the process of a current feedback control, the difference between the motor current reference value and the detected motor current value grows large resulting in an erroneous determination that an abnormal current is flowing in the motor.
As a countermeasure against this phenomenon, the applicant of the present invention proposed a method of determining whether the difference between the motor current estimate value computed by applying a mathematical model on the motor current reference value and the detected motor current value that has been detected exceeds the predetermined allowed value, and determining that the driving system has failed when the difference exceeds the allowed value (refer to Japanese Laid Open Patent Publication JP2002-234457).
And when the detected value of the steering torque has turned out to be an abnormal value, for example, when the motor wiring is connected in the reverse direction to the normal state, the direction of the steering assist force that should be primarily given by the motor is not coincided with the direction of the steering assist force actually given by the motor. As a result, the detected value of the steering torque turns out to be an abnormal value, and the detected motor current value also turns out to be an abnormal value. Consequently, a fail-safe control is activated and the steering assistance is terminated.
As a countermeasure against this phenomenon, it is proposed a method of determining that the detected value of the steering torque is an abnormal value when the detected value of the steering torque exceeds the predetermined value or the change ratio of the steering torque exceeds the predetermined value, or that the detected motor current value is an abnormal value when the motor current exceeds the predetermined value or the change ratio of the motor current exceeds the predetermined value and taking the predetermined measures required in the case of detecting an abnormality (refer to Japanese Laid Open Patent Publication JP2003-237609).
According to the former method of determining the failure of the driving system described above, when the motor current detecting circuit for detecting the motor current has failed, as the reversal of the motor current direction repeatedly occurs and the time during which the difference between the motor current estimate value and the detected motor current value exceeds the allowed value is short, it is difficult to determine any abnormality, and therefore another detection means is necessary for detecting whether the motor current detection circuit is normal or abnormal.
And according to the latter method of determining failure, it is difficult to set the predetermined reference value (threshold value) for determining the abnormality of motor current or change rate in motor current, and depending on the setting of the predetermined reference value, a determination of normal condition or that of failure may be given.
The object of the present invention is to provide an electric power steering device capable of solving the problem described above and detecting correctly the failure of the current detection circuit.